sender check: connections and messages counted apart; downtime estimated when supervisord is silent (stage 2b)

Following the design commit df96690.

- Two columns (owner, 2026-10-14): sender_check_days.messages (migration
  0009, not deployed) holds the messages deferred mid-message, from
  cleanup's warning; deferred stays the connections smtpd turned away.
  AddSenderCheckDeferred takes which; the counter passes cleanup's as a
  message. The card has "Connections turned away" and "Messages deferred";
  it warns on either within 24 hours (one mark, as before).
- Supervisord silent at start: postfixStarted asks again every 2 s for 30 s
  (postfixState: started, running, known). With no answer the downtime is
  the time since the alive mark - an upper bound - booked with estimated = 1
  (sender_check_days.estimated, MAX over a day); the card prefixes such a
  period's time with "≈ " and its note says what ≈ means. Before, such a
  start counted nothing and the number the owner decides by read low.
- Help topic sender-check and the log topic word for word as the mockup
  (help.go).

Tests: store (connections and messages apart, a period with an estimated
day); logtail (cleanup's warnings are messages); cmd/panel (supervisord
silent: the time since the mark, estimated); view fixture and the Health
page at four columns with ≈; e2e reads the row's two counts (panel down:
connections; panel hung: messages). Schema at 0009 with the columns.

schema-migrations row 9, CHANGELOG [Unreleased], audit plan lines ticked
(two), roadmap 28/62.
Evidence: go vet ./... clean; go test of store, view, cmd/panel ok locally,
logtail but for the Windows-only test; the server run follows before the
push.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/logtail/sendercheck.go

@@ -48,7 +48,7 @@ import (
 type SenderCheckStore interface {
 	// AddSenderCheckDeferred counts and saves the place in the log reached,
 	// in one transaction.
-	AddSenderCheckDeferred(at time.Time, n int64, place string) error
+	AddSenderCheckDeferred(at time.Time, n int64, place string, message bool) error
 	SenderCheckDeferredAt() (time.Time, error)
 	SenderCheckCountedPlace() (string, error)
 }
@@ -118,7 +118,7 @@ func (c *deferredCounter) handle(line string, now time.Time, catchingUp bool, pl
 	if program == "cleanup" {
 		// A message cleanup could not finish because the check did not reply.
 		if strings.HasPrefix(text, c.broken) {
-			c.count(at, catchingUp, place)
+			c.count(at, catchingUp, place, true)
 		}
 		return
 	}
@@ -137,13 +137,14 @@ func (c *deferredCounter) handle(line string, now time.Time, catchingUp bool, pl
 			c.failed = make(map[string]bool)
 		}
 		c.failed[pid] = true
-		c.count(at, catchingUp, place)
+		c.count(at, catchingUp, place, false)
 	}
 }
 
-// count adds one deferred message at at, the line ending at place, unless it
-// is a line read again.
-func (c *deferredCounter) count(at time.Time, catchingUp bool, place Place) {
+// count adds, at at, one connection turned away - or, with message, one
+// message deferred in the middle - the line ending at place, unless it is a
+// line read again.
+func (c *deferredCounter) count(at time.Time, catchingUp bool, place Place, message bool) {
 	if catchingUp && c.countedAlready(at, place) {
 		return
 	}
@@ -151,7 +152,7 @@ func (c *deferredCounter) count(at time.Time, catchingUp bool, place Place) {
 		c.last = at
 	}
 	c.counted = place
-	if err := c.st.AddSenderCheckDeferred(at, 1, place.String()); err != nil {
+	if err := c.st.AddSenderCheckDeferred(at, 1, place.String(), message); err != nil {
 		log.Printf("log-tailer: sender check statistics: %v", err)
 	}
 }